The present invention relates to an improved system for coating and corrosion proofing metal cans, such as seamless aluminum as well as tin plate, tin free and blackplate steel cans, which can be drawn and ironed on a draw and iron press located at the initial sections or stations of a typical continuous can production line as well as an improved coated product produced thereby. Prior art can washing devices that have been incorporated in such can manufacturing lines are shown, for example, in U.S. Pat. No. 3,262,460, issued July 26, 1966, and U.S. Pat. No. 3,291,143, issued Dec. 13, 1966. The can washer devices of these patents generally comprise a series of successive wash, rinse and drying stations and intermediate blowoff stations wherein residual oils and greases present on the surfaces of metal containers or cans such as those manufactured on conventional draw and iron presses are first removed from the cans preparatory to the further treatment and handling of the cans. As the cans pass through a washer device of the type shown in U.S. Pat. No. 3,262,460 they are ordinarily subjected to what is generally referred to as a post-cleaning or post-surface treatment by an acid wash, which usually employs an inorganic chromate-phosphate solution in order to make the outside and inside surfaces of the can more receptive to coatings as well as printing inks.
There have been a number of problems coincident with the use of inorganic chromate-phosphate solutions or similar acid washes, in addition to the fact that they are expensive. For example, when such materials are employed in can washes there must be a continual monitoring of the amount of materials used because the overall effectiveness of such acid washes is dependent to a large extent upon the amount of chemical agents in solution. There are also environmental problems involved in disposing of waste chromate-phosphate solutions, in that, in the past it has been a prerequisite during disposal to neutralize the same such as by treating the material with SO.sub.2 gas prior to emptying the waste liquors containing such acids or chromate-phosphate solutions into the conventional plant sewers or other waste disposal systems.
On the other hand, despite the disadvantages of the acid wash and complex can handling involved, it has been generally heretofore impossible in a commercial can production line to avoid such a post-cleaning or post-surface can treatment, because the can surfaces had to be sized by such treating processes in order to make them receptive to the coatings and/or printing and decorating inks demanded by can customers. Such acid wash or caustic etching operations were particularly significant in can manufacturing lines, where drawn and ironed metal cans were processed, because the walls of such cans are relatively smooth and slippery and unless appropriately pretreated may not be receptive to coatings and/or printing and decorating inks. For example, the outside wall surfaces of such cans are usually highly polished and buffed due to the action of the ironing dies on such wall surfaces.
A discussion of the can cleaning and coating problems with which this invention is particularly concerned would not be complete without mention being made of the necessity of having appropriate surface coatings applied to the metal cans to avoid attack by the contents. This problem is particularly acute in the case of non-alcoholic and carbonated beverages which today represent a substantial amount of the overall can manufacturing and filling business throughout the world.
The contents of the containers have raised other problems, in that, it has been largely impossible to date to avoid the use of separate and special inside and outside can coatings and curing operations therefor, which can be quite different from each other and require separate, distinct and expensive multi-stage operations. These coatings are required in order for the cans to be utilized as food or beverage containers while complying with various State and Federal laws and/or regulations with respect to different beverages. All of this, in turn, means that the cost of manufacturing and handling the cans is substantially raised each time a separate can coating operation is required.
The instant invention is concerned with minimizing the aforesaid can processing problems and effecting substantial cost reductions by utilizing an improved can body treatment system. A feature of the proposed system is that it involves equipment which is generally compatible with and can be readily integrated with much of the equipment of today's standard metal can processing lines, such as one involving drawn and ironed aluminum or steel cans or impact extruded aluminum metal cans. A particular advantageous and significant feature of the instant container processing system concerns the fact that the coating operation proposed can utilize but a single water based, preferably water soluble and at least water dispersible, organic and preferably polymeric coating having hydrophilic characteristics and excellent wettability properties. For a further discussion of the desirability and need for such a system, reference may be made to an article appearing at pages 23 and 24 of the Mar. 27, 1974, issue of Chemical Week magazine.
The coating in accordance with the invention can be applied to both the inside and outside surfaces of a can simultaneously in a single operation. In one advantageous embodiment of the invention the metal cans are pretreated with a moisture layer that improves and promotes the interfacial surface tension of the metal surfaces and thus enhances the adhesion of the coating material to the metal can. During this coating operation, a common, readily flowable, low viscosity, organic coating material is simultaneously applied in a flooding manner to both inside and outside surfaces of a can such that all of the exterior and interior can surfaces can be said to be simultaneously deluged or innundated with the coating. The manner of coating material application advantageously assists in orienting and positioning the cans relative to the coating applicators during the correlated movement of the cans through the coating zone and no special holders are required as in the case of U.S. Pat. No. 3,353,515, issued Nov. 21, 1967. Subsequent to the application and curing of the coating on the cans, the cans may either be immediately palletized for delayed processing or they can be subsequently continuously processed while being relatively corrosion proofed.